The process of papermaking involves the formation of a web of fibers on a papermachine wire from a slurry of treated wood pulp, water removal from the fibers in the press roll section and in the dryer section of the papermaking machine, and final treatment of the paper by calendaring, chemicals and/or heat. In a typical papermaking process, the web from the press roll section contains about 32 to 35 wt. % solids. The solids may include wood pulp fibers and various additives such as sizing, binders, fillers, pigments and the like. The wet web is then passed through a series of internally heated rolls or steam-filled cylinders whereby the web is dried to about 94% solids content by weight. The number of dryer cylinders is determined by the amount of water to be evaporated based on a typical evaporation rate of about 2 pounds per hour per square foot of total dryer surface.
In the dryer section of the papermachine, water is removed from the web mainly by evaporation. Typically, the wet web is alternately contacted on its opposite sides with a series of hot cylindrical surfaces to heat the web to a temperature whereby water will evaporate from the web to a desired solids content.
Once dried, the paper or paperboard is generally further treated to improve various properties such as smoothness, gloss, wet strength and folding endurance. This subsequent treatment may include adjusting the moisture content of the dried web, densification on high pressure rolls, calendaring and/or heat treating the paper or paperboard product.
Various problems have persisted in the drying of paper webs on large, high-capacity paper machines. For example, drying of paper or paperboard products remains a high energy, capital intensive operation. Hence, the industry is constantly seeking to develop newer and more energy efficient drying techniques. Such drying techniques include high-intensity drying techniques whereby high temperatures and mechanical pressures are applied to the web during drying. Examples of currently used high-intensity drying techniques include press drying, impulse drying, and thermal/vacuum drying. However, the use of high temperature dryers and/or impulse dryers has led to additional problems such as delamination of linerboard products.
Furthermore, in the presently used high-intensity dryers, the paper may shrink by as much as 5 to 6% in the cross direction, i.e. in the direction perpendicular to the direction of travel through the papermaking machine. For wide sheets of paper or paperboard, such a shrinkage rate results in a significant reduction in the overall paper production rate.
Accordingly, even with the new high-intensity drying techniques, there still remains a need to further improve the drying of paper and paperboard products so as to reduce energy costs and reduce paper shrinkage while at the same time not adversely affecting any of the other physical properties of the finished paper or paperboard product.
Uneven drying is another problem which has persisted in drying paper webs. It has been known to apply moisture to portions of a web in the drying section of a paper making machine in order to prevent dry streaks and to assure uniform dryness across the width of the web. The weight and moisture irregularity of the fiber web before drying, irregularities in the heat transfer from the cylinders, edge effects and variations in the ventilation of the papermaking machine all tend to cause nonuniform drying in the cross-direction of the web. Such nonuniformity of drying has a negative effect on paper quality and may also result in increased waste. U.S. Pat. No. 4,378,639 to Walker and U.S. Pat. No. 4,474,643 to Lindblad propose solutions to the problem of uneven drying across the width of the web. These involve the periodic spraying of water on the web in selected areas across the width of the web where low moisture or dry streaks have been detected. Since only the areas of the web requiring moisture adjustment are sprayed, and only when streaking is sensed, the methods do not relate to improvement in the water removal rate.
It is therefore an object of the invention to improve the drying of paper and paperboard products without adversely affecting the physical properties of the finished product.
Another object of the invention is to increase the water removal rate from a paper or paperboard web in the drying portion of a paper and paperboard-making process.
Still another object of the invention is to provide a method for drying paper and paperboard whereby delamination and shrinkage of the paper and paperboard product is reduced.
Other objects and benefits of the invention will be evident from the ensuing discussion and appended claims.